1. Field of the Invention
The invention is in the broad field of heat exchangers primarily in industrial heating where conservation of fuel has made it increasingly important to improve the overall efficiency of extracting energy from the stack gases resulting from combustion. The industrial apparatus for this energy extraction is known as a recuperator.
2. Description of the Prior Art
A common form of prior art recuperator is a multi-walled cylinder with the flue gas passing upwardly through a central cylinder and combustion air passing through two or more of the surrounding annular passageways. The cylinder wall separating the flue gas and combustion air is the primary heat transfer surface. With a double pass on the air side of the heat exchanger, the outer air passage, although counter current to the inner air passage, serves primarily to utilize the incoming air as insulation to the primary heat transfer occurring in the inner air passage. The inner air passage relies on counterflow convective heat transfer from the primary heat transfer surface to the combustion air. Where the flue gas cylinder is unobstructed throughout its length, it can be described as having no core. A core describes a device inserted in the flue gas passage of a recuperator for the purpose of enhancing heat transfer. With no core, heat transfer is accomplished primarily by radiation from the hot gases and convection due to the flow of gas past the outer wall surrounding the flue gas passage. Radiation from the flue gas is a function of the gas composition, the dimensions of the flue column and the temperature of the flue gas. Heat transfer by convection is a function of the velocity of flue gas relative to the heat transfer surface.
An example of a primarily convective core is U.S. Pat. No. 3,859,040, Jan. 7, 1975, invented by Shefsiek and Cone and assigned to the assignee of the present invention. An example of a reradiating core is U.S. Pat. No. 3,886,976, June 3, 1975, invented by Kardas et al. and assigned to Institute of Gas Technology. Other prior art patents revealed in a novelty search are:
U.s. pat. Nos.: 1,128,146 Jacobs; 1,172,144 Keairnes; 1,858,665 Gibson; 2,356,530 Pflock; 2,462,838 Brierly; 2,726,681 Gaddis et al.; 2,733,908 Graham; 2,910,276 Escher; 3,518,908 Singh; 3,595,299 Weishaupt et al., and 3,886,976 Kardas.